What is it?: A polariscope is a gemological instrument that tests
for a gemstone being double or single refractive, and will allow
us to find the various crystal axis of the stone. Which is where
is got its name: Polar-scope..the a scope to see the poles or
axis of the stone. Now, if you remember the section on the dichroscope,
the use of plane polarized light allows us to see
the different colors of light being transmitted by a gemstone.
The polariscope, however, allows us to actually see the path that
those beams are taking through the stone. By knowing that the
stone is double refractive, we can use the optic interference
figure to actually find the various optical directions
that the light is traveling through the stone, and thereby make
identifications based on this information.

Let's clarify this concept. As we found
with the dichroscope, single refractive gemstones do not break
the light beam up into various colors. Meaning that one beam in
and one beam out. There are relatively few gemstones that are
single refractive, and identification of those stones is fairly
easy owing to their rarity. There are a lot of double refractive
gemstones though. Some break up a beam of light into two beams...like
ruby, sapphire and emerald...while others break up a beam of light
into three separate beams...like tanzanite and andalusite. What
the polariscope does is allow us to see the different directions
that the light is traveling through the stone. How? By allowing
us to see the optic interference figure that the
light beam creates as it travels through the stone.

It is truly a defining moment in the life
of any gemologist when they first are able to find and identify
their first optic interference figure. The first
time you actually see a quartz bulls-eye you will
(if you are like me) sit back in your chair for a minute and contemplate
the wonder of not only seeing this figure, but realizing that
what you are seeing is the path of a beam of light through a gemstone.
This is a truly wonderful event...even if some of the old timer's
are too far removed from their own event to remember. I remember...and
it was 28 years ago that I found my first uniaxial interference figure. Admittedly, it did not rival seeing my son
take his first breath....but it was an astounding event nonetheless.

There is a lot of technical gemology that
we do not have room to cover here regarding the polariscope and
what it is actually telling you. But if you will take a few minutes
and learn what it is saying, perhaps later we can
spend more time and understand the why it is saying
it. But for now, learn the what of the polariscope,
it will allow you to make gemstone identifications from a very
early time in your study. And we will take up the why in the course notes of the International School of Gemology. But for now, let's learn
the parts of the polariscope:

Why it works: As
stated above, the polariscope works by transmitting plane
polarized light into a gemstone. When you turn both of
the polaroid plates to the dark position, a single
refractive gemstone will not change the path of that light, and
the stone will remain dark in all positions. With double refractive
stones, however, the stone will actually change the path of the
light, thereby changing the direction of the wave of light, and create a plane of light that is no longer polarized.
It is this beam of light that has had its direction of travel
changed that makes the stone turn light to dark as you rotate
it between the polaroid plates. This is how the polariscope works.
But wait...it does more. With the use of a conoscope you can actually see that beam of light travel through the stone
in what we referred to above as an optic interference figure. The conoscope is a simple glass sphere that has no internal
strain to give false strain readings as shown with the amber below.
With the conoscope, and some people can do the same thing with
a 10x loupe, you can actually see the optic interference figure,
which will tell you whether a stone is uniaxial or biaxial.

You can purchase a professional quality polariscope at a special visitor price in the YourGemologist Gemology Equipment Store by clicking on this link:YourGemologist's Polariscope.

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